The invention relates to protection circuitry for protecting circuits against transients such as electrostatic discharge (ESD). In particular, the invention relates to circuitry for protecting analog bipolar circuits, including BJT and BICMOS based circuits against voltage transients.
Analog circuits typically display sensitivity to excessive voltage levels. Transients, such as ESD can cause the voltage handling capabilities of the analog circuit to be exceeded, resulting in damage to the analog circuit. Clamps have been devised to shunt current to ground during excessive voltage peaks.
One of the difficulties encountered in designing such protection circuitry is that the specifications for these clamps have to fit within a relatively small design window that, on the one hand, takes into account the breakdown voltage of the circuit being protected. Thus, the clamp must be designed such as to be activated below the breakdown voltage of the circuit that is to be protected. At the same time, the design window is limited by the latchup phenomenon to ensure that the clamp is not conducting under normal operating conditions. The latchup voltage must exceed the normal operating voltage of the protected circuit.
Typical protection clamps employ avalanche diodes such as zener diodes to provide the bias voltage for the base of a subsequent power bipolar junction transistor (BJT). In a prior art claim comprising a reverse coupled BJT that acts as an avalanche diode, the BJT has a defined reverse breakdown. A voltage pulse supplied to the input that exceeds the breakdown voltage of the BJT, causes an avalanche effect in the BJT and results in current flow from the emitter to the collector when the reverse breakdown voltage of the BJT is exceeded. The current drives the base of a power BJT and switches the power BJT on, thus biasing the base to switch on the power BJT. Once the power BJT switches on, collector/emitter current is shunted to ground. Instead of a reverse coupled BJT, one or more zener diodes could be used instead. For example, in a five volt power supply circuit where the power BJT is a 10 volt BJT, two 3.5 volt zener diodes could be used to replace the reverse coupled BJT. Until the power BJT switches on, the voltage across the clamp, which is also the voltage applied to the protected circuit, increases during a voltage peak such as a human body discharge that may typically be 120-125% percent of the final holding voltage.
A problem with the prior art solution is the excessive time delay before BJT starts conducting. This causes an over voltage as high as 20% and more. This short term overload voltage may last for 10-30 ns., and can cause burn out or other damage to the protected circuit.
According to the invention there is provided an overvoltage protection circuit for protecting an input of an analog bipolar circuit, which comprises a first bipolar junction transistor connected between an input of the analog bipolar circuit and ground; a reverse-coupled bipolar junction transistor, wherein its emitter is connected to the input, and its collector is connected to the base of the first bipolar junction transistor, and, further comprising, a capacitor connected to the base of the reverse coupled bipolar junction transistor to inject current into the base.
The first transistor acts as a switch for shunting current to ground, while the reverse-coupled transistor serves to switch on the first transistor when the reverse breakdown voltage of the reverse-coupled transistor is exceeded. The capacitor lowers the reverse-breakdown voltage of the reverse-coupled transistor.
The first transistor and reverse-coupled transistor are preferably NPN bipolar junction transistors.
More generally, the invention provides an overvoltage protection circuit, comprising a first transistor for shunting current to ground, a reverse-coupled transistor connected to the first transistor to force the first transistor into conduction when the reverse-coupled transistor is forced into conduction, and a capacitor connected to the reverse-coupled transistor to more rapidly cause the reverse-coupled transistor to conduct. The capacitor causes the reverse-coupled transistor more rapidly to conduct by reducing the break-down voltage of the reverse-coupled transistor. This is achieved by injecting current into the base of the reverse-coupled transistor. The first transistor may be a BJT or a field effect transistor.
Further, according to the invention, there is provided a method of improving an overvoltage protection circuit for an input to a protected circuit, wherein the overvoltage protection circuit includes a first transistor for shunting current to ground, and a reverse-coupled transistor connected to the first transistor to switch on the first transistor, the method comprising the step of lowering the reverse-breakdown voltage of the reverse-coupled transistor when a voltage transient occurs.